Reactions, classes homogeneous

As discussed already in Chapter 7, redox reactions constitute a second class of geochemical reactions that in many cases proceed too slowly in the natural environment to attain equilibrium. The kinetics of redox reactions, both homogeneous and those catalyzed on a mineral surface are considered in detail in the next chapter, Chapter 17, and the role microbial life plays in catalyzing redox reactions is discussed in Chapter 18. 

The electron formed as a product of equation (2.5) will usually be received (or collected ) by an electrode. It is quite common to see the electrode described as a sink of electrons. We need to note, though, that there are two classes of electron-transfer reaction we could have considered. We say that a reaction is heterogeneous when the electroactive material is in solution and is electro-modified at an electrode which exists as a separate phase (it is usually a solid). Conversely, if the electron-transfer reaction occurs between two species, both of which are in solution, as occurs during a potentiometric titration (see Chapter 4), then we say that the electron-transfer reaction is homogeneous. It is not possible to measure the current during a homogeneous reaction since no electrode is involved. The vast majority of examples studied here will, by necessity, involve a heterogeneous electron transfer, usually at a solid electrode. 

Catalysts are usually subdivided into homogeneous and heterogeneous classes. Homogeneous catalysts are soluble in the reaction media. Heterogeneous catalysts make up the bulk of the catalyst market they are the solid catalysts that can be a support material, such as alumina, silica, or silica alumina, but more often some metal salt is added to a formed catalyst support. 

Soluble polymers have attracted recent attention in catalysis and combinatorial chemistry.1 3 When used in catalysis by organometallic compounds, soluble polymer ligands offer the following advantages The reaction is homogeneous in nature and separation of catalysts can be easily achieved by filtration or precipitation. We have now developed a new class of polymer ligands based on fluoroacrylate-arylphosphine copolymers for catalysis in periluorocarbon solvents and supercritical C02 (scC02). 

It is probably necessary to make a primary operational distinction of reaction classes based on the phase (or phases) of matter involved thus (1) homogeneous, liquid phase (2) gas phase (3) solid phase (4) heterogeneous. A basic subclassification distinguishes between reactions in which the reactants are chemically different from the prodncts, as in equations (1) and (2), and reactions in which the reactants and prodncts involve the same chemical species, as in equations (3) and (4) when (N4) = ( N4). Eqnations (1) and (2) are examples of cross electron-transfer reactions (or cross-reactions), while eqnations (3) and (4) are examples of self-exchange electron-transfer reactions when (N4) = ( N4). More generally, subclassifications of the primary classes are commonly based on energy or free energy considerations such as ... 

Outer-sphere electron transfer is one of the simplest reaction types because no bonds are broken or formed. It is therefore not surprising that this class of reactions was the subject of early kinetic theories. More than 30 years ago Marcus (1965) derived a predictive theory for the rate constants of os redox reactions in homogeneous and heterogeneous systems. A didactic introduction was later given by the same author (Marcus, 1975), and Sutin (1986) reviewed modern refinements of the theory. 

Engineering of organic synthesis analysis and design of reactors for the important classes of reactions, mainly homogeneous, two-phase catalytic, two-phase noncatalytic, and three-phase catalytic and noncatalytic... 

Table 9.6 Selected chiral syntheses belonging to different classes of reactions using homogeneous catalysts... 

Nitrogen-directed C—H activation to form C—C bonds like aryl-aryl is a relatively recent class of reactions. In 2008, Nakamura and coworkers presented the first report of an iron-catalyzed C—H arylation reaction—a homogeneous iron catalysis featuring C—C bond formation via C—H activation. As an overall synthetic transformation, it was a formal nucleophilic displacement of the ort/ o-hydrogen atom by... 

Oxo Synthesis. Ad of the synthesis gas reactions discussed to this point are heterogeneous catalytic reactions. The oxo process (qv) is an example of an industriady important class of reactions cataly2ed by homogeneous metal complexes. In the oxo reaction, carbon monoxide and hydrogen add to an olefin to produce an aldehyde with one more carbon atom than the original olefin, eg, for propjiene ... 

PdClJ , Rh(III) and Ru(III) act as homogeneous catalysts for reduction of FeClj by molecular hydrogen ° °. The kinetics of all three activation reactions fall into Class I. The Arrhenius parameters are... 

Most of the major developments in homogeneous transition metal-catalyzed hydrocyanation since the early 1980s have been reviewed 62 67 considerable effort in this area has focused on enantioselective reactions.68 In this regard, the emphasis has been on complexes of the group 10 metals (particularly Ni and Pd) incorporating new classes of phosphine, phosphite, and phos-phinite ligands. 

Vinyl ethers constitute a third class of monomers which have been cationically polymerized in C02. While fluorinated vinyl ether monomers such as those described in Sect. 2.1.2 can be polymerized homogeneously in C02 because of the high solubility of the resulting amorphous fluoropolymers, the polymerization of hydrocarbon vinyl ethers in C02 results in the formation of C02-insoluble polymers which precipitate from the reaction medium. The work in this area reported to date in the literature includes precipitation polymerizations and does not yet include the use of stabilizing moieties such as those described in the earlier sections on dispersion and emulsion polymerizations (Sect. 3). 

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